Yes, you can leave the playa for supplies. No, there won't be any services. While there may be someone out there who offers you something in the way of food, the whole radical self reliance thing kind of dictates that you'll be wanting/needing to bring your own supplies.
My experiences in getting there early have been largely to work and get things done. If you've got time to party, you probably came too early.
Depending on the crew or the project's work schedule, you may have some time to relax here and there, or share a drink with friends, but mostly it's been a function of getting things put together and ready to go.
I've never had any bad experiences with anyone from DPW, but I've always been busy working on my own projects, and they've been taking care of their own business.